1. Field of the Invention
The present invention relates generally to a method for transmitting and receiving broadcast service data in a wireless communication system. In particular, the present invention relates to a method for transmitting and receiving broadcast service data in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
A cellular mobile communication system is a typical wireless communication system. The mobile communication system uses a multiple access scheme to simultaneously communicate with a plurality of users. The multiple access scheme used in the mobile communication system typically includes a Time Division Multiple Access (TDMA) scheme and a Code Division Multiple Access (CDMA) scheme. With the rapid progress of communication technology, the CDMA mobile communication system has been evolving from a system for mainly supporting voice communication into a system capable of transmitting high-speed packet data.
However, due to limited code resources, the CDMA mobile communication system has difficulty in transmitting an increasing amount of multimedia data. Accordingly, there is a need for a multiple access scheme capable of identifying an increased number of users and transmitting an increased amount of data to the users. A multiple access scheme proposed to meet the need is the Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The OFDMA scheme identifies users with a plurality of orthogonal subchannels, and transmits data over the orthogonal subchannels.
For high-speed data transmission, a cellular system using the OFDMA scheme (hereinafter referred to as an “OFDMA cellular system”) has been proposed. Research into the OFDMA scheme is being conducted by an Institute of Electrical and Electronic Engineers (IEEE) 802.16d standardization committee to provide a high-speed wireless Internet service. The 802.16 system supports a high-speed service at a data rate higher than that of a 3rd generation (3G) mobile communication system, and implements the high-speed service using a transmission scheme such as an Orthogonal Frequency Division Multiplexing (OFDM) scheme and advanced antenna techniques such as a Multiple Input Multiple Output (MIMO) antenna technique and a smart antenna technique in addition to several physical layer techniques used in the 3G mobile communication system.
However, the conventional 802.16 high-speed wireless communication system transmits data to individual mobile stations on a unicast basis, and recently, a technique of transmitting and receiving data for a broadcast service provided in the 3G mobile communication system tends to be applied to the 802.16 system.
A description will now be made of a process of processing a broadcast service transmitted from a physical layer in the current mobile communication system wherein the broadcast service is provided through a downlink channel.
FIG. 1 is a signaling diagram illustrating a process of processing a broadcast service transmitted from a physical layer in a mobile communication system wherein the broadcast service is provided through a downlink channel.
It is assumed in FIG. 1 that a particular base station transmits broadcast service data for a broadcast service A and a broadcast service B. In this case, mixed data 100 in which the broadcast service A and the broadcast service B coexist as broadcast traffic. In step 110, the mixed data 100 for the two different broadcast services A and B is subject to channel coding. Thereafter, in step 120, a channel-coded signal 102 for the two different broadcast services is subject to interleaving and modulation. In step 130, an interleaved and modulated signal 104 for the two different broadcast services is mapped to a transmission channel through a transmission process for the broadcast services. In this manner, a radio signal for the broadcast services A and B is transmitted over transmission channels 106a, 106b and 106c. The radio signal is transmitted per predetermined transmission unit.
The foregoing method may have the following problems when mixed broadcast service data is transmitted. First, because broadcast information A and broadcast information B were encoded together, even though a mobile station desires to receive one of the broadcast information A and the broadcast information B, it must receive both a radio signal for the broadcast service A and a radio signal for the broadcast service B and then decode the received radio signals. Second, a transmission time period for the broadcast traffic is too short, making it difficult to maximally improve transmission performance by using time diversity.
Generally, the broadcast service unidirectionally transmits downlink signals to a plurality of mobile stations. Therefore, the broadcast service can rarely undergo downlink power control, and in order to overcome a fading radio channel, a transmission time period should be longer than a coherence time of a radio channel to obtain sufficient time diversity. In this case, however, sufficient time diversity cannot be obtained. A description thereof will be made with reference to FIG. 2.
FIG. 2 is a timing diagram for a description of a relationship between a change in channel fading and transmission data. Each of transmission unit-based frames shown in an upper part of FIG. 2 represents a downlink packet transmitted from a base station. Herein, because a description of the broadcast service is being made, it will be assumed that the packet is a mixed radio signal for different broadcast services as described in connection with FIG. 1. That is, mixed radio signals in each of which different broadcast services coexist are transmitted in the order of i, i+1, i+2, i+3, i+4, i+5, . . . , as shown in FIG. 2. In addition, it will be assumed that an amplitude response characteristic 200 of a time-varying fading channel has a curve shown in a lower part of FIG. 2. An (i+3)th radio frame transmitted from a time 210 to a time 212 has a very high frame error probability. That is, the radio signal cannot have sufficient time diversity, thereby deteriorating reception performance.
In other words, in FIG. 2, as the (i+3)th frame of a transmission broadcast signal suffers a so-called “deep fade” in which a radio channel is rapidly deteriorated by fading, a received signal-to-noise ratio is very low. As a result, a frame error probability of the transmission broadcast signal is very high. A third problem of the transmission method of FIG. 1 is as follows. Because broadcast signals A and B are encoded and modulated together, it is impossible to allow them to have their own unique performances. That is, assuming that the broadcast information A is information for a broadcast service for the overall service area of the base station and the broadcast information B is information for only a particular area with a good channel environment in the entire service area of the base station, it is impossible to enable the broadcast information A and B to have their own unique performances because the broadcast information A and the broadcast information B are simultaneously encoded and modulated as described with reference to FIG. 1.